The invention relates to a method of manufacturing a color television display tube having a color selection electrode in which in at least one step of the method a supporting frame of the color selection electrode must be accurately positioned on at least three mandrels present in the face plate of the tube.
During the manufacture a color television display tube, one of the most critical steps is establishing the spacing between the color selection electrode and the face plate with very small tolerances.
The introduction of the so-called In-Line technique in which the three electron gun systems are arranged in one plane beside each other and luminescent strips having alternating colors are formed on the screen provided a certain relaxation of tolerances, because in the direction of the strips the tolerances for the landing point of the electron beam on the display screen could be slightly widened. However, in the direction of the raster scan lines the tolerances remained so that during tube manufacture, and especially during assembly of the color selection electrode, utmost care must be taken when positioning the color selection electrode with respect to the face plate.
With the increase in brightness introduced recently, the requirements on the landing accuracy of the electron beams on the associated luminescent strips were further increased. As a result of this, the permissible tolerances in the distance between the inner surface of the face plate and the outer surface of the mask sheet were further restricted, which necessitated an even higher accuracy of the position of the color selection electrode in the face plate.
During tube manufacture, several adjusting steps are performed to align the parts as accurately as possible with respect to each other. For example, in an early step of a known manufacturing process a supporting frame with holders connected thereto is inserted into the face plate. The holders typically include apertures which snap into engagement with corresponding mandrels provided in the face plate glass. This supporting frame is then welded to a pre-bent mask sheet which is positioned as accurately as possible. The sheet had been laid on a spacing jig, placed on the inner surface of the face plate, before the supporting frame was inserted. It has been found that the supporting frame was often not held correctly in the face plate during welding, because the holders had not been placed firmly on the mandrels. As a result the mask sheet would not be correctly positioned on the supporting frame, but would be welded in a slightly oblique position.
In a much later step of the tube manufacturing process, the finished color selection electrode, comprising the supporting frame and the mask sheet, is inserted into the face plate and the holders are snapped into engagement with the mandrels. An additional holder is then snapped onto a respective face plate mandrel and this holder is also welded to the supporting frame. This additional holder increases color selection electrode's mechanical stability. During the welding of the additional holder it is also necessary for the color selection electrode and its already-connected holders to be correctly positioned on the mandrels. It is known that when the mandrels are slightly rough, the holders will not slide correctly on the mandrels. The suggestion has been made to polish the mandrels and to punch the corresponding apertures in the holders so as to be slightly conical to provide a better sliding surface. However, such steps are very expensive in production line operations.
It has also been established that during flow coating when the individual exposures are made for the red, green and blue phosphors, the snapping of the color selection electrode onto the mandrels in the face plate does not occur sufficiently accurately. Differences for each individual exposure operator occur.
Also during the final positioning of the color selection electrode in the face plate before assembling the tube, special care is necessary. This last step determines how accurately the electron beam of a gun will land on its associated luminescent strips in the finished tube. Although errors in electrode positioning cause beam deviations which are hardly noticeable in the center of a display screen, considerable deviations are visible in the proximity of the corners.
From the prior art, jolting tables are known for finding defective contacts during the manufacture of electronic circuits, and jolting transporters are known for use during the manufacture of electronic components for transport. In the manufacture of color television display tubes, jolting of the tube envelope is also known for shaking loose particles out of the mask area so they do not disturb the displayed picture. Typically loose particles such as welding spatters, glass splinters and organic substances occur. These cause clear shadows or color defects on the display screen during operation of the color television display tube if they are located in front of the slots in the color selection electrode.
Tubes having color purity defects caused by improper frame positioning during manufacture can be corrected afterward only with difficulty.